1. Field of the Invention
The present invention relates to method and system for a Radio Frequency (RF)-based identification, tracking and locating of objects.
2. Background of the Related Art
RF-based identification and location-finding systems for determination of relative or geographic position of objects are generally used for tracking single objects or groups of objects, as well as for tracking individuals. Conventional location-finding systems have been used for position determination in the open outdoor environment. RF-based, Global Positioning System (GPS), and assisted GPSs are typically used. However, conventional location-finding systems operate in the UHF band and suffer from certain inaccuracies when locating the objects in closed environments, as well as outdoors.
The indoor and outdoor location inaccuracies are due mainly to the physics of RF propagation, in particular, due to losses/attenuation of the RF signals, scattering and reflections. Accordingly, there is a need for a method and a system for object identification and location-finding, which is accurate in both indoor and outdoor environments.
Conventional identification and tracking-location systems have not used the narrow bandwidth signals for accurate RF tracking and locating. Previous developments have taught away from using such narrow bandwidth signals for RF identification, tracking and location of objects. The narrow bandwidth signals have been deemed impractical because accurate identification and location of objects requires extremely accurate distance measurements. The distance measurements are used in triangulation/trilateration or virtual triangulation calculations.
Conventional systems are restricted to single offices for point-to-point communications. By way of further example, a hospital building is full of various metal objects, including medical equipment in metal enclosures, metal cabinets, tables, or the like. If a 900 MHz RF signal is transmitted in this type of space, the RF wavelength is only 30 cm, and since most of these objects are significantly larger than 30 cm, a large portion of the RF energy will be reflected. Also, at 900 MHz, the RF energy will be significantly attenuated as the signal passes through internal walls or building floors, reducing the operating range. These indoor effects make signals at 900 MHz (or higher) frequencies ineffective in object locating in any reasonably complex indoor environment due to the RF signal losses caused by the potential reflectors and the signal losses through the walls.
A wireless RF-based system is subjected to “free space” losses associated with the attenuation of the signal. In addition to the “free space” losses, other propagation losses in real-world RF-based systems include reflection, scattering, diffraction, shadowing, refraction, and absorption. The limitation of using UHF high frequencies is that they are highly attenuated in passing through walls (See, e.g. http://www.stanford.edu/class/ee359/lecture2.pdf) and are scattered or absorbed by conductive and non-conductive objects.
The UHF high frequencies attenuation is caused by the small wavelengths as compared to objects normally found in buildings and in an urban environment. Therefore, higher frequency (UHF) RF tracking and locating systems are limited to operating in two regimes. The first regime, used primarily by GPS, is effective in the open space where direct unobstructed line of sight to three or more satellites broadcasting the signals can be established. This allows for good triangulation and object location. However, it does not work as well in closed environments, such as urban corridors, forested land, canyons and adverse weather conditions. This regime does not work inside buildings.
The second regime is active RFID, which is sometimes used for location within buildings. However, the attenuation and scattering of the signals limits the operating range to a hundred feet. An active RFID reader in each room may be required. As the operating distance increases between the master unit/reader and a slave unit/tag, there is more reflection, scattering and attenuation of the direct path RF signal energy. This decreases the power ratio between the Direct Line of Sight (DLOS) signal, and all other indirect path signals.
On the other hand, increasing the power of the transmitted signal does not solve the “free space” loss and other propagation loss problems. While higher transmitted signal power increases the RF link budget and consequently the operating range, it does not affect the propagation of the RF signal energy. The higher transmitted signal power causes more reflection, scattering and attenuation of the direct path RF signal energy. The power of all indirect path signals also increases proportionally. Thus, increasing the transmitted signal power does not extend the operating range without reducing the accuracy.
Existing RFTL (RF Track-Locate) systems almost all make use of “active RFID” technology or “GPS” technology. In either case, they operate in the UHF band of radio frequencies, defined as those between 300 and 3000 MHz.
At these relatively high radio frequency bands, temporally narrow (i.e., a very short pulse) ranging signals can be sent from the Master unit to the slave tags or between the slave tags. These signals are relatively unambiguous in shape when they are received, and that allows well-defined TOA (Time-Of-Arrival) and DTOA (Differential Time-Of-Arrival) measurements and hence, relatively accurate time and distance measurements to be made.
Thus, ranging accuracy is usually thought to depend on a very short duration RF pulse. It is well-understood from Fourier analysis that an RF pulse is composed of very many frequency components. The sharper or narrower in time the pulse is, the more frequency components it has; it is thus described as “broadband” or having “broad-bandwidth”. This broadband signal is usually only permitted in the UHF or higher frequency bands where more unused bandwidth is available.
However, the use of higher RF frequencies introduces other problems. More specifically, it is well-known that such signals are heavily attenuated or absorbed by many materials, including ordinary walls of buildings. For this reason, GPS-based RFTL systems are limited to use out of doors. Similarly active RFID-based RFTL systems, when used in a building have very limited range (up to 100 feet or 200 feet), because of absorption or attenuation in the walls of a room. This means that an RFTL system used to locate people or objects in a building needs multiple fixed readers (or master units), perhaps as many as one in each room, thereby increasing its inherent expense.
Further, even when the higher frequency RF ranging signals are not absorbed, they are prone to being scattered by metallic and non-metallic objects in their path. This leads to a plethora of different ranging signals arriving at the slave tag (or back to the Master unit) at slightly different times from an unscattered or direct signal; this is manifested as noise in the received ranging signal and is analogous to the “ghosts” that were seen with early television signals.
Accurate distance measurement is determined by accurate measurement of the transit time of an RF ranging signal (from a satellite to a GPS receiver, or from an active RFID reader to a tag, and back to the reader). An accurate distance measurement can be achieved with a narrowest possible ranging signal. The reason for the narrow bandwidth signals not having been used in conventional systems is that, the narrower is the RF signal band, the broader the signal temporally. Also, VHF band locating signals have not been developed, in part, because of the governmental agency spectrum restrictions, such as, for example, Federal Communication Commission (FCC) requirements. The FCC has limited signals in the VHF and the lower frequency bands to very narrow bandwidths (less than 30 KHz).
As a result, there is a need in the art for a method and system for object identification and location-finding, which uses narrow bandwidth signals in VHF or lower frequencies. There is also a need for a method and system for object identification and location-finding that is effective when only two devices (Master and target) are present in the environment, or when a signal, such as a GPS signal, cannot be received, for example, inside a mall, urban canyons, etc.